boards: arm: qemu_cortex_r5: add chosen entry for OCM

Add the 'chosen' entry for the OCM whose declaration is being added to the
SoC's device tree. This enables the linker command file to properly place
data explicitly assigned ocm_bss/ocm_data sections.

Signed-off-by: Immo Birnbaum <Immo.Birnbaum@weidmueller.com>